IRF logical ports can be configured with physical 10Gbps member ports so you should use SFP+ Transceivers (if you have Fiber Optic links), DAC Cables (if you want use them and Switches are near each others) or XGT copper (RJ45) ports (for Ethernet Cat. 6 certified cabling)...in any case you are going to have 10Gbps interfaces involved to form IRF ports.

[*] Note that both SFP+ slots are also compatible with slower SFP 1Gbps Transceivers (so not capable of 10Gbps as SFP+ ones), those SFP Transceivers are not supported for being used on IRF member ports to form IRF logical ports).